1 files changed, 61 insertions, 0 deletions
diff --git a/lib/mystring/join.cc b/lib/mystring/join.cc
new file mode 100644
index 0000000..435765f
--- /dev/null
+++ b/lib/mystring/join.cc
@@ -0,0 +1,61 @@
+#include "mystring.h"
+#include <string.h>
+
+// This "join" class relies on one fairly obscure detail in the C++
+// standard: temporaries are destructed only after the entire
+// "full-expression" has completed.  That is, if the sequence:
+// f(f(f(x))) creates three temporary objects, the inner objects are
+// destroyed only after the execution has completed.  This allows us
+// to build a complete linked-list on the stack.  Tricky, but efficient!
+
+struct tmpitem
+{
+  const char* str;
+  unsigned len;
+};
+
+mystringrep* mystringjoin::traverse() const
+{
+  // At first glance, a recursive implementation would be the most logical
+  // way of doing this, but it turned out to be a significant loss.  This
+  // method traverses the pointer chain to first determine the length, and
+  // then to do the actual copying.
+
+  // Note the use of do/while loops to avoid a test at the head of the loop
+  // which will always succeed (there is always at least one node or item).
+  unsigned count = 0;
+  const mystringjoin* node = this;
+  do {
+    ++count;
+  } while((node = node->prev) != 0);
+
+  // The use of a temporary array avoids re-traversing the pointer
+  // chain, which is a slight performance win.
+  tmpitem items[count];
+  
+  unsigned length = 0;
+  node = this;
+  tmpitem* item = items;
+  do {
+    unsigned l = node->rep ? node->rep->length : strlen(node->str);
+    length += l;
+    item->str = node->str;
+    item->len = l;
+    ++item;
+  } while((node = node->prev) != 0);
+
+  // Since the chain is constructed such that the last item is the
+  // first node, the string gets constructed in reverse order.
+  mystringrep* rep = mystringrep::alloc(length);
+  char* buf = rep->buf + length;
+  item = items;
+  do {
+    unsigned l = item->len;
+    buf -= l;
+    memcpy(buf, item->str, l);
+    ++item;
+  } while(--count != 0);
+
+  rep->buf[length] = 0;
+  return rep;
+}